Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metals from polluted environments and nuclear waste, as well as to produce industrially relevant metals, such as rare earth ions. Selectivity arises from the choice of an extractant amphiphile, dissolved in the organic phase, which interacts preferentially with the target metal ion. Little is known about the molecular mechanism of the extractant-mediated process of ion transport, which involves a change in the ion coordination from a fully hydrated ion in the bulk aqueous phase to an ion coordinated to extractants and enclosed within a reverse micelle of extractants in the bulk organic phase. By utilizing a method to switch the extraction on and off, an intermediate state of the extraction process can be immobilized at the aqueous-organic interface. X-ray reflectivity and fluorescence measurements are then used to investigate the nanoscale structure of the ion-extractant complex for both strontium and rare earth erbium ions. Contrary to the common expectation that monolayers of extractant amphiphiles at the aqueous-organic interface underlie the transport process, as observed for strontium, the measurements on samples containing Er reveal a counter-intuitive interfacial arrangement of extractants in the intermediate state, namely an inverted bilayer of extractants, whose headgroups complex and shield ions from the organic phase. This novel molecular arrangement is a 2-dimensional condensed phase of ion-extractant complexes. EXAFS measurements of the bulk organic phase indicate that Er ion extraction proceeds by desorption of ion-extractant complexes from the interface into the bulk phase.

Insights from X-ray surface scattering on the interfacial transport of ions from aqueous to organic phases

250th National Meeting (2015)

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